Computational study of subcritical response in flow past a circular cylinder

Flow past a circular cylinder is investigated in the subcritical regime, below the onset of Benard-von Karman vortex shedding at Reynolds number Re-c similar or equal to 47. The transient response of infinitesimal perturbations is computed. The domain requirements for obtaining converged results is discussed at length. It is shown that energy amplification occurs as low as Re = 2.2. Throughout much of the subcritical regime the maximum energy amplification increases approximately exponentially in the square of Re reaching 6800 at Re-c. The spatiotemporal structure of the optimal transient dynamics is shown to be transitory Benard-von Karman vortex streets. At Re similar or equal to 42 the long-time structure switches from exponentially increasing downstream to exponentially decaying downstream. Three-dimensional computations show that two-dimensional structures dominate the energy growth except at short times.